Polyolefin resins having bimodal molecular weight distributions and/or bimodal composition distributions are desirable in a number of applications. Resins including a mixture of a relatively higher molecular weight polyolefin and a relatively lower molecular weight polyolefin can be produced to take advantage of the increased strength properties of higher molecular weight resins and articles and films made therefrom, and the better processing characteristics of lower molecular weight resins.
Bimetallic catalysts such as those disclosed in U.S. Pat. Nos. 5,032,562 and 5,525,678, and European Patent EP 0 729 387, can produce bimodal polyolefin resins in a single reactor. These catalysts typically include a non-metallocene catalyst component and a metallocene catalyst component which produce polyolefins having different average molecular weights. U.S. Pat. No. 5,525,678, for example, discloses a bimetallic catalyst in one embodiment including a titanium non-metallocene component which produces a higher molecular weight resin, and a zirconium metallocene component which produces a lower molecular weight resin.
As stated in U.S. Pat. No. 6,995,109, controlling the relative amounts of each catalyst in a reactor, or the relative reactivity of the different catalysts, allows control of the bimodal product resin. Other background references include EP 0 676 418, WO 98/49209, WO 97/35891, and U.S. Pat. No. 5,183,867.
Bimetallic catalysts are also disclosed in, for example, U.S. Pat. Nos. 7,199,072, 7,141,632, 7,172,987, 7,129,302, 6,964,937, 6,956,094, and 6,828,394.
Hydrogen gas is often used in olefin polymerization to control the final properties of the polyolefin, where the hydrogen gas acts as a chain transfer agent during polymerization. Single metal based catalyst using hydrogen as a chain transfer agent may produce a polymerization product with relatively predictable molecular weight and molecular weight distribution. Further, changes in reactor hydrogen concentration may additionally be used to alter the molecular weight and molecular weight distribution of the resulting polymer.
Bimetallic catalysts such as those described in the patents listed above, however, typically include catalyst components having different hydrogen responses (each having a different reactivity toward hydrogen). For example, a first catalyst component may have a higher response to changes in reactor hydrogen concentration than a second catalyst component. Thus, in contrast to single metal based catalysts, a change in reactor hydrogen concentration may affect molecular weight, molecular weight distributions, and other properties of the resulting bimodal polymer when using a bimetallic catalyst. Owing to the differing hydrogen response of the components in the bimetallic catalyst, control of polymer properties is considerably more complicated and less predictable, as there is an additional independent variable (a second catalyst having a different hydrogen response) affecting polymerization dynamics.
A need exists for bimetallic catalyst systems having predictable and controllable responses to changes in reactor hydrogen concentration.